On a teenager's life timeline, the 17th birthday is pointlessly wedged between sweet 16th and legalizing 18th celebrations. While it affords no adolescent soirees or lottery tickets, the middle milestone does impact Friday nights at the movies. Twenty years ago on Oct. 5, Henry & June hit theaters as the first film to hold an NC-17 rating. Unlike an R-rated flick that would force a parent to be your date, there is no wiggle room with these titles  moviegoers under age 17 are not permitted in the audience. But how come this particular Motion Picture Association of America (MPAA) rating is so young? And why is it not commonplace to hear that baritone "rated NC-17" voice bookend movie trailers at the theater or on television?

Since 1968, the MPAA has relied on a parent panel to create advisory film ratings for moviegoers. As chairwoman of the unit's Classification and Ratings Administration (CARA), Joan Graves oversees the 10-member board that grades hundreds of films each year, from G to NC-17. Prior to 1990, films displaying patently adult material received an X. Graves recalls the climate surrounding the NC-17 switch as somewhat puzzling. "I remember asking [longtime MPAA President] Jack [Valenti] one time why that happened, why they trademarked every rating but the X," Graves says. "He said it didn't seem necessary at the time because that rating was meant just for adults." (See a Q&A on the 40th anniversary of movie ratings.)

But in 1990, the connotations behind the X rating were changing: the pornography industry was gradually appropriating the X sign in order to add some cinematic cachet to its productions. Lacking legal claim over the ratings, the MPAA watched adult-video titles snap up XXX ratings, stigmatizing its own code with the taint of smut. To remedy this, NC-17 was instituted on Sept. 27 as the new notation for "content appropriate only for an adult audience." Originally marked as an X film, the movie of a love triangle between Henry and June Miller and Anaïs Nin subsequently aired on Oct. 5 with the first NC-17 marking in film history. (See a 1992 story on what was to become of the NC-17 movie rating.)

Twenty years later, the rating has largely avoided attaining a pornographic frisson. Sure, the 1995 film Showgirls (1995) was a spectacle of Las Vegas lust, tagged for nudity and erotic sexuality, as well as graphic language and sexual violence. But Man Bites Dog (1992) received the same classification exclusively for graphic violence. More often than not, Graves' board gives the NC-17 review on those grounds. "To this day, when the films are submitted to us, we do give more NC-17's for violence than we do for sex," Graves says. "The content of the film actually has turned out that way."

While the descriptors of each MPAA rating remain fixed, the context of a film's content is in flux. "Patently adult" content in 2010 deviates from the standards set in 1968, 1990 or any other year. Computer-generated-imagery effects give directors more control over a film's visuals, leaving an R-rated movie like Saving Private Ryan (1998)  and its sequences of graphic war language and violence  on the fringe. "I think that perhaps a decade or two decades ago, that might have been considered NC-17," Graves says. "And I think in today's world, the parents on the board looked at that and thought there would be parents that  even though it was very, very poignantly violent  would want their teens to see it." (See TIME's photo essay "Tom Hanks: America's Chronicler-in-Chief.")

Filmmakers have historically had their own qualms about their movie hitting the market as NC-17. In 1990, some newspapers refused to run ads for NC-17 films and some video stores opted to not carry those titles. Currently, Graves and the CARA board deliver assessments on the same day that a movie is viewed. If the motion picture company agrees with the rating, they can accept the judgment and have it certified for distribution. For filmmakers displeased with the designated rating, their picture can be edited and resubmitted to achieve a new designation. Otherwise, there is an appeals board. Over the past decade, 21 NC-17 films have been submitted via that route, with Zack and Miri Make a Porno (2008) standing as the only film overturned to an R rating.

From what Graves hears from filmmakers, her board's grades are tied to higher distribution levels; MPAA member companies often prefer to have a rating because it allows their movies access to a wider range of theaters. On the flip side, independent nonmember companies focusing on specific markets can release their films under the "not rated" classification.

For films going the rated route, NC-17 still shakes the psyche of some shareholders. But the past two decades have brought plenty of business highlights, from The Dreamers (2003) to Lust, Caution (2007). "All it takes is a very good movie that has some wide appeal to come out as an NC-17, and it puts the whole rating on the map again," Graves says. "It just depends on who's making the decisions on how they want to market the product."